The present invention relates to a method and a device for producing preforms for molding an advantageous base geometry for a later blow-molding process.
Preforms are injection-molded blanks of at least one thermoplastic material that are used in blow-molding machines for the production of stretch-blow-molded plastic containers.
For the customary production of preforms described according to this invention, polymer raw material is plasticized and subsequently forced at high pressure into a single- or multi-cavity mold.
This produces preforms according to FIG. 1, which geometrically consist substantially of a neck and shaft region and a domed base end, and are hollow on the inside due to the insertion of a core in the mold. The neck region is shaped in such a way that it may for example be configured so as to be re-closable with a screw cap. The neck region, however, does not undergo any further change during the blow-molding process. By contrast, the shaft region and the domed base end are inflated at elevated temperatures to form hollow bodies, whereby the polymer is stretched and at the same time considerably solidified. Therefore, in conjunction with the core geometry, the preform regions to be deformed are geometrically responsible for the quality of the bottle that is later obtained.
Since the mold usually represents the greatest investment in a production system, great value is attributed to it operating efficiently. Thus, the preform, the outer skin of which is in direct contact with the intensively cooled steel of the mold, and consequently solidifies quickly there, is demolded without any damage and without mechanical deformation, in order that the mold is ready for the next production cycle without any time being lost.
With the customary high-speed production cycles, a considerable residual heat remains in the interior of the preform wall, which leads to a re-heating, whereby the preform can soften again and crystallize out, which makes it become unusable.
It is therefore indispensable to continue intensively cooling the preform after demolding in simpler mold parts, known as cooling sleeves, during several production cycles.
The preform, as it is represented in FIG. 1, corresponds to the current state of the art, in which it is inevitable that the wall thicknesses of the preform have similar wall thicknesses, particularly in the region of the domed base end and the shaft. If the material sets prematurely due to thinner wall thicknesses in the gating region, shrinking in the cooling phase due to follow-up pressure on the melt cannot be avoided, with an effect on the entire preform including the neck region, which as a consequence leads to undesired sink marks in critical regions of the preform.
The preform geometry, as it is shown in FIG. 2 and the advantages of which are still to be explained below, therefore cannot be produced by the known injection-molding process, since the wall thickness is much thinner in the region of the domed preform end, and consequently in the vicinity of the gate, than in the further progression of the preform shaft, and consequently sink marks as a result of premature setting of this thin region, particularly in the neck region, can no longer be avoided.